Carbon Dioxide Gas-Phase Reduction Device and Carbon Dioxide Gas-Phase Reduction Method

ABSTRACT

The efficiency of a carbon dioxide reduction reaction at a reduction electrode is improved. An oxidation tank including an oxidation electrode, a reduction tank which is adjacent to the oxidation tank and to which carbon dioxide is supplied, and a gas reduction sheet located between the oxidation tank and the reduction tank are included, the gas reduction sheet is a sheet in which an ion exchange membrane and a reduction electrode are laminated, the ion exchange membrane is located on the oxidation tank side, the reduction electrode is located on the reduction tank side, is connected to the oxidation electrode by a lead, and conduct conducts a reduction reaction of the carbon dioxide by a current flowing through the lead.

TECHNICAL FIELD

The present invention relates to a carbon dioxide gas phase reduction apparatus that induces a carbon dioxide reduction reaction by light irradiation or induces an electrolytic reduction reaction of carbon dioxide to improve the efficiency of the reduction reaction, and belongs to the technical fields of fuel generation technology and solar energy conversion technology.

BACKGROUND ART

In the related art, a technology for reducing carbon dioxide has attracted attention from the viewpoint of prevention of global warming and stable supply of energy, and has been actively studied in recent years. As a technique for reducing carbon dioxide, there are a reduction apparatus utilizing artificial photosynthesis for applying light energy such as sunlight to reduce carbon dioxide, and an electrolytic decomposition apparatus for applying electric energy from the outside to reduce carbon dioxide.

RELATED ART 1

FIG. 5 is a diagram illustrating a configuration of a carbon dioxide reduction apparatus in a related art. The reduction apparatus is an apparatus for a reduction reaction of carbon dioxide at a reduction electrode by irradiating an oxidation electrode with light (artificial photosynthesis device).

An oxidation tank 1 includes an oxidation electrode 2 a and an aqueous solution 3, and a reduction tank 4 includes a reduction electrode 5 a and an aqueous solution 3. The oxidation electrode 2 a and the reduction electrode 5 a are submerged in the aqueous solution 3. The oxidation electrode 2 a is formed from, for example, a compound exhibiting photoactivity or redox activity, such as nitride semiconductor, titanium oxide, amorphous silicon, ruthenium complex, or rhenium complex. The reduction electrode 5 a is formed from, for example, copper, platinum, gold, indium, tungsten (VI) oxide, copper (II) oxide, or porous metal complex having a metal ion and an anionic ligand. The aqueous solution 3 is, for example, a sodium hydroxide aqueous solution, a potassium hydroxide aqueous solution, a potassium hydrogen carbonate aqueous solution, a sodium hydrogen carbonate aqueous solution, a potassium chloride aqueous solution, or a sodium chloride aqueous solution.

Helium is supplied to the oxidation tank 1 via a tube 8. Carbon dioxide is constantly supplied to the reduction tank 4 via the tube 8. An ion exchange membrane 6 is sandwiched between the oxidation tank 1 and the reduction tank 4. The ion exchange membrane 6 is, for example, a cation exchange membrane called Nafion (trade name), and is a perfluorocarbon material composed of a hydrophobic Teflon skeleton made of carbon-fluorine and a perfluoro side chain having a sulfonic acid group.

Protons generated by an oxidation reaction of water in the oxidation tank 1 are transported to the reduction tank 4 via the ion exchange membrane 6. The oxidation electrode 2 a and the reduction electrode 5 a are connected by a lead 7. A light source 11 is closely disposed to face the oxidation electrode 2 a. The light source 11 is, for example, a xenon lamp, a pseudo solar light source, a halogen lamp, a mercury lamp, sunlight, or a combination thereof.

The light source 11 irradiates the oxidation electrode 2 a with light. Using holes and electrons generated by irradiating the oxidation electrode 2 a with light, oxygen is generated by an oxidation reaction of water in the oxidation tank 1, hydrogen is generated by a reduction reaction of protons, and carbon monoxide, methane, ethylene, methanol, ethanol, and formic acid are generated by a carbon dioxide reduction reaction in the reduction tank 4.

RELATED ART 2

FIG. 6 is a diagram illustrating another configuration of a carbon dioxide reduction apparatus in a related art. The reduction apparatus is an apparatus for an electrolytic reduction reaction of carbon dioxide at a reduction electrode (electrolytic reduction reaction apparatus).

An oxidation tank 1 includes an oxidation electrode 2 b and an aqueous solution 3, and a reduction tank 4 includes a reduction electrode 5 b and an aqueous solution 3. The oxidation electrode 2 b and the reduction electrode 5 b are submerged in the aqueous solution 3. The oxidation electrode 2 b is formed from, for example, platinum, gold or silver. The reduction electrode 5 b is formed from, for example, copper, gold, platinum, silver, zinc, palladium, gallium, indium, tin, or cadmium. The aqueous solution 3 is, for example, a sodium hydroxide aqueous solution, a potassium hydroxide aqueous solution, a potassium hydrogen carbonate aqueous solution, a sodium hydrogen carbonate aqueous solution, a potassium chloride aqueous solution, or a sodium chloride aqueous solution.

Helium is supplied to the oxidation tank 1 via a tube 8. Carbon dioxide is constantly supplied to the reduction tank 4 via the tube 8. An ion exchange membrane 6 is sandwiched between the oxidation tank 1 and the reduction tank 4. The ion exchange membrane 6 is, for example, a cation exchange membrane called Nafion, which is a perfluorocarbon material composed of a hydrophobic Teflon skeleton made of carbon-fluorine and a perfluoro side chain having a sulfonic acid group.

Protons generated by an oxidation reaction of water in the oxidation tank 1 are transported to the reduction tank 4 via the ion exchange membrane 6. The oxidation electrode 2 b, a reference electrode 10, and the reduction electrode 5 b are connected to a power supply 9 by a lead 7. An external voltage can be applied between the reference electrode 10 and the reduction electrode 5 b to flow a current between the oxidation electrode 2 b and the reduction electrode 5 b.

When the external voltage is applied, oxygen is generated by an oxidation reaction of water in the oxidation tank 1, hydrogen is generated by a reduction reaction of protons, and carbon monoxide, methane, ethylene, methanol, ethanol, and formic acid are generated by a carbon dioxide reduction reaction in the reduction tank 4.

CITATION LIST Non Patent Literature

-   Non Patent Literature 1: Yoshio Hori, 2 others, “Formation of     Hydrocarbons in the Electrochemical Reduction of Carbone Dioxide at     a Copper Electrode in Aqueous Solution”, Journal of the Chemical     Society, 85(8), 1989, p. 2309-p. 2326. -   Non Patent Literature 2: Heng Zhong, 2 others, “Effect of KHCO3     Concentration on Electrochemical Reduction of CO2 on Copper     Electrode”, The Electrochemical Society 164 (9), 2017, p. F923-p.     F297 -   Non Patent Literature 3: Satoshi Yotsuhashi, 6 others, “CO2     Conversion with Light and Water by GaN Photoelectrode”; Japanese     Journal of Applied Physics, 51, 2012, p. 02BP07-1-p. 02BP07-3 -   Non Patent Literature 4: Hiroshi Hashiba, 4 others, “Selectivity     Control of CO2 Reduction in an Inorganic Artificial Photosynthesis     System”; Applied Physics Express, 6, 2013, p. 097102-1-p. 097102-4

SUMMARY OF THE INVENTION Technical Problem

In the related arts 1 and 2, carbon dioxide injected from the tip of the tube 8 to the vicinity of the bottom of the reduction tank 4 is dissolved in the aqueous solution 3 and reaches the reduction electrodes 5 a and 5 b, and is reduced on the surfaces of the reduction electrodes 5 a and 5 b. To improve the efficiency of the carbon dioxide reduction reaction, it is necessary to increase the concentration of carbon dioxide and decrease the diffusion resistance of carbon dioxide in the reduction tank 4. However, when carbon dioxide is supplied to the aqueous solution 3 via the tube 8, the concentration of carbon dioxide that can be dissolved in the aqueous solution 3 has a limit, and the diffusion resistance of carbon dioxide in the aqueous solution 3 is large. Thus, the amount of carbon dioxide supplied to the surfaces of the reduction electrodes 5 a and 5 b has a limit.

Therefore, an object is to improve the efficiency of the carbon dioxide reduction reaction on the reduction electrodes 5 a and 5 b by increasing the amount of carbon dioxide supplied to the surfaces of the reduction electrodes 5 a and 5 b through the increase of the concentration of carbon dioxide in the reduction tank 4 and decrease of the diffusion resistance of carbon dioxide.

Further, in the related arts 1 and 2, the reduction electrodes 5 a and 5 b come into contact with the aqueous solution 3 in the reduction tank 4, which results in proceeding of a deterioration reaction in which the reduction electrode material is eluted as complex ions or reacts with dissolved oxygen in the aqueous solution 3 to become copper oxide. As a result, the reaction area of the reduction electrodes 5 a and 5 b is decreased, and the life of the carbon dioxide reduction reaction is decreased. To improve the life of the reduction reaction of carbon dioxide, it is necessary to prevent the contact between the reduction electrodes 5 a and 5 b and the aqueous solution 3, but the contact cannot be prevented in the apparatus in which the reduction electrodes 5 a and 5 b are submerged in the aqueous solution 3.

Therefore, an object is to prevent the reaction area from decreasing due to the deterioration of the reduction electrodes 5 a and 5 b by preventing the reduction electrodes 5 a and 5 b from contacting the aqueous solution 3 in the reduction tank 4, thereby improving the life of the carbon dioxide reduction reaction.

The present invention has been made in view of the above circumstances, and an object of the carbon dioxide gas phase reduction apparatus according to the present invention is to improve the efficiency of the carbon dioxide reduction reaction at the reduction electrode, and to improve the life of the carbon dioxide reduction reaction by preventing the reduction electrode from deteriorating due to the reaction with the aqueous solution. An object of the carbon dioxide gas phase reduction method according to the present invention is to suppress the decrease in the reaction area due to solid deposition on the surface of the reduction electrode, and to further improve the life of the carbon dioxide reduction reaction.

Means for Solving the Problem

The carbon dioxide gas phase reduction apparatus of the present invention includes: an oxidation tank including an oxidation electrode; a reduction tank which is adjacent to the oxidation tank and to which carbon dioxide is supplied; and a gas reduction sheet located between the oxidation tank and the reduction tank, wherein the gas reduction sheet is a sheet in which an ion exchange membrane and a reduction electrode are laminated, the ion exchange membrane is located on a side of the oxidation tank, and the reduction electrode is located on a side of the reduction tank, is connected to the oxidation electrode by a lead, and conducts a reduction reaction of the carbon dioxide by a current flowing through the lead.

The carbon dioxide gas phase reduction apparatus further includes a light source for irradiating the oxidation electrode with light.

The carbon dioxide gas phase reduction apparatus further includes a power source connected to the lead.

In the above-mentioned carbon dioxide gas phase reduction apparatus, the oxidation electrode includes an n-type semiconductor.

The carbon dioxide gas phase reduction method of the present invention is performed in a carbon dioxide gas phase reduction apparatus, the carbon dioxide gas phase reduction apparatus including an oxidation tank including an oxidation electrode, a reduction tank adjacent to the oxidation tank, and a gas reduction sheet located between the oxidation tank and the reduction tank, the gas reduction sheet being a sheet in which an ion exchange membrane and a reduction electrode are laminated, the ion exchange membrane being located on a side of the oxidation tank, the reduction electrode being located on a side of the reduction tank and connected to the oxidation electrode by a lead, the gas phase reduction method including: filling the oxidation tank with an aqueous solution; supplying carbon dioxide to the reduction tank, and flowing an electric current through the lead to cause a reduction reaction of the carbon dioxide at the reduction electrode.

In the above-mentioned carbon dioxide gas phase reduction method, the aqueous solution is any one of a potassium hydroxide aqueous solution, a rubidium hydroxide aqueous solution, and a cesium hydroxide aqueous solution.

Effects of the Invention

According to the present invention, the carbon dioxide gas phase reduction apparatus according to the present invention allows the decrease in the diffusion resistance of carbon dioxide near the surface of the reduction electrode, and the improvement of the efficiency of the carbon dioxide reduction reaction at the reduction electrode. In addition, the reduction electrode can be prevented from deteriorating due to reaction with the aqueous solution, and the life of the carbon dioxide reduction reaction can be improved. According to the carbon dioxide gas phase reduction method according to the present invention, it is possible to suppress the decrease in the reaction area due to the deposition of solids on the surface of the reduction electrode, and to further improve the life of the carbon dioxide reduction reaction.

Further, according to the present invention, as a current collecting method for the reduction electrode, the metal plate is pressed against the outer periphery of the electrode and the metal mesh is pressed against the surface of the reduction electrode, which further improves the efficiency and life of the carbon dioxide reduction reaction.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram illustrating a configuration of a carbon dioxide gas phase reduction apparatus according to Example 1.

FIG. 2 is a diagram illustrating a configuration of a reaction system of an electroless plating method.

FIG. 3 is a diagram illustrating a configuration of a carbon dioxide gas phase reduction apparatus according to Example 5.

FIG. 4 is a diagram illustrating a configuration of a carbon dioxide gas phase reduction apparatus according to Comparative Example 2.

FIG. 5 is a diagram illustrating a configuration of a carbon dioxide reduction apparatus in a related art.

FIG. 6 is a diagram illustrating another configuration of a carbon dioxide reduction apparatus in the related art.

FIG. 7 is a diagram illustrating an example of a conductive structure at a reduction electrode.

FIG. 8 is a diagram illustrating an example of the conductive structure at the reduction electrode.

FIG. 9 is a diagram illustrating an example of the conductive structure at the reduction electrode.

DESCRIPTION OF EMBODIMENTS

Hereinafter, an embodiment of the present invention will be described with reference to the drawings. The present invention is not limited to this embodiment, and may be modified without departing from the spirit of the present invention.

The shape of current collecting means described in the present embodiment is not limited to the shape and structure described in the present embodiment, and may be any conductive structure as long as it does not hinder the diffusion of carbon dioxide. For example, sponge-like metal having excellent gas diffusibility or carbon cloth used as a gas diffusion electrode may be pressure-bonded to a reduction electrode. Even in that case, the same effect can be obtained.

SUMMARY OF THE INVENTION

The carbon dioxide gas phase reduction apparatus according to the present invention has a configuration in which carbon dioxide in the gas phase is directly supplied to a gas reduction sheet obtained by directly forming a reduction electrode on an ion exchange membrane. With this configuration, it is possible to improve the efficiency of a carbon dioxide reduction reaction on the reduction electrode.

Further, the carbon dioxide gas phase reduction apparatus according to the present invention is configured to bring only gas-phase carbon dioxide into contact with the reduction electrode. This configuration can prevent deterioration of the reduction electrode due to a reaction with an aqueous solution, and improve the life of a carbon dioxide reduction reaction.

In the carbon dioxide gas phase reduction apparatus according to the present invention, as a configuration and method for connecting the reduction electrode and a lead, a configuration in which a metal plate is pressed against the outer periphery of the reduction electrode and a configuration in which a metal mesh is pressed against the surface of the reduction electrode are used. With this configuration, it is possible to achieve sufficient current collection between the reduction electrode and the lead, and further improve the efficiency and life of the carbon dioxide reduction reaction.

Further, in the carbon dioxide gas phase reduction method according to the present invention, any one of a potassium hydroxide aqueous solution, a rubidium hydroxide aqueous solution, and a cesium hydroxide aqueous solution is used as an aqueous solution with which an oxidation tank is filled. By this method, it is possible to suppress a decrease in the reaction area due to solid deposition on the surface of the reduction electrode, and to further improve the life of the carbon dioxide reduction reaction.

Example 1 Configuration of Apparatus

FIG. 1 is a diagram illustrating a configuration of a carbon dioxide gas phase reduction apparatus 100 according to Example 1. This carbon dioxide gas phase reduction apparatus 100 is an improved invention of the related art 1, and is an apparatus for carbon dioxide reduction reaction at a reduction electrode by irradiating an oxidation electrode with light (artificial photosynthesis apparatus). Hereinafter, it is abbreviated as the gas phase reduction apparatus 100.

As illustrated in FIG. 1, the gas phase reduction apparatus 100 includes an oxidation tank 1 and a reduction tank 4, which are formed by dividing an internal space inside a housing into two parts. Specifically, the gas phase reduction apparatus 100 includes the oxidation tank 1 filled with an aqueous solution 3, an oxidation electrode 2 a composed of a semiconductor or metal complex inserted into the oxidation tank 1, and the reduction tank 4 adjacent to the oxidation tank 1 and filled with carbon dioxide or a gas containing carbon dioxide.

A gas reduction sheet 20 is disposed between the oxidation tank 1 and the reduction tank 4. The gas reduction sheet 20 is obtained by laminating a reduction electrode 5 made from a metal layer on an ion exchange membrane 6. The ion exchange membrane 6 is disposed on the oxidation tank 1 side, and the reduction electrode 5 is disposed on the reduction tank 4 side. The oxidation electrode 2 a and the reduction electrode 5 are connected by a lead 7 (the lead 7 is also connected to the oxidation electrode 2 a), and a light source 11 for operating the gas phase reduction apparatus 100 is disposed to face the oxidation electrode 2 a.

Further, as illustrated in FIG. 7, a metal wire 13 is connected to the reduction electrode 5 by a silver paste 14 and fixed with an adhesive applied onto the silver paste 14. The contact area between the reduction electrode 5 and the current collector (metal wire 13 and silver paste 14) is about 0.87 cm², which corresponds to about 14% of the area of the reduction electrode 5.

The oxidation electrode 2 a is formed from, for example, a compound exhibiting photoactivity or redox activity, such as a nitride semiconductor, titanium oxide, amorphous silicon, a ruthenium complex, or a rhenium complex.

The reduction electrode 5 is formed from, for example, copper, platinum, gold, silver, indium, palladium, gallium, nickel, tin, cadmium, an alloy thereof, silver oxide, copper oxide, copper (II) oxide, nickel oxide, indium oxide, tin oxide, tungsten oxide, tungsten oxide (VI), copper oxide, or a porous metal complex having an anionic ligand and a metal ion.

The aqueous solution 3 is, for example, a potassium hydrogen carbonate aqueous solution, a sodium hydrogen carbonate aqueous solution, a potassium chloride aqueous solution, a sodium chloride aqueous solution, a sodium hydroxide aqueous solution, a potassium hydroxide aqueous solution, a rubidium hydroxide aqueous solution, or a cesium hydroxide aqueous solution.

The ion exchange membrane 6 is, for example, a cation exchange membrane called Nafion, which is a perfluorocarbon material composed of a hydrophobic Teflon skeleton made of carbon-fluorine and a perfluoro side chain having a sulfonic acid group.

The light source 11 is, for example, a xenon lamp, a pseudo solar light source, a halogen lamp, a mercury lamp, sunlight, or a combination thereof.

Gas Reduction Sheet

When carbon dioxide in the gas phase is directly supplied to the reduction electrode 5 without passing through the aqueous solution 3, protons also need to be directly supplied to the reduction electrode 5 without passing through the aqueous solution 3. Therefore, by forming the reduction electrode 5 directly on the ion exchange membrane 6, a gas reduction sheet 20 in which the ion exchange membrane 6 and the reduction electrode 5 are integrated is produced. As the producing method, an electroless plating method, which is a technique of forming a metal film on a material without using an external power source, was adopted. The procedure for producing the gas reduction sheet 20 by the electroless plating method will be described below.

Nafion was used for the ion exchange membrane 6, and copper was used for the reduction electrode 5. To improve the adhesion of copper to the ion exchange membrane 6, one side of the ion exchange membrane 6 was polished with abrasive paper. Further, to improve the proton mobility of the ion exchange membrane 6, the ion exchange membrane 6 was immersed in boiling nitric acid for one hour and in boiling pure water for one hour. FIG. 2 illustrates a reaction system of an electroless plating method used as a method for producing the gas reduction sheet 20.

A tank 31 and a tank 32 are filled with a plating solution 41 and a reducing agent 42 shown in Table 1, respectively. The tank 31 and the tank 32 are separated by the ion exchange membrane 6.

Table 1 shows the components of a plating solution and a reducing agent used for preparing the gas reduction sheet by the electroless plating method.

TABLE 1 Plating solution Reducing agent CuSO₄•5H₂O 3.5 g/L NaBH₄ 10 vol. % KNaC₄H₄O₆•H₂O 34.0 g/L  NaOH 7.0 g/L Na₂CO₃ 3.0 g/L

The polished surface of the ion exchange membrane 6 was placed on the plating solution 41 side. NaBH₄, which is the main component of the reducing agent 42, is a polar compound, and thus permeates the ion exchange membrane. Then, at the interface between the plating solution 41 and the ion exchange membrane 6, a redox reaction (BH₄ ⁻+4OH⁻→BO₂ ⁻+2H₂O+2H₂+4e⁻, Cu²⁺+2e⁻→Cu) occurred to precipitate copper, whereby the gas reduction sheet 20 having the reduction electrode 5 formed on the ion exchange membrane 6 was obtained. Further, by changing the plating solution 41 and the reducing agent 42 to different chemicals, metals such as Ni, Au, Ag, Pd, and Sn can be formed, and a metal oxide or a metal complex can be formed by subjecting the metal to an oxidation reaction or a substitution reaction.

As to the gas reduction sheet 20, copper particles are stacked on the ion exchange membrane 6, and the reduction electrode 5 with a structure having voids between the particles is formed. However, in this structure, even if the lead is brought into contact with one local part of the reduction electrode, current cannot be collected from all the metal particles constituting the reduction electrode. Under the plating solution concentration conditions in this example, the thickness of the reduction electrode was about 200 nm, and the reduction electrode had a porosity of about 5%. The thickness of the reduction electrode 5 can be controlled by changing the plating solution concentration conditions, the plating reaction time, and the reaction temperature. When the thickness is decreased, the reduction electrode 5 has an island-like structure, so that electrons cannot move between the islands and the reaction does not proceed. Further, when the thickness is increased, the diffusion of carbon dioxide in the reduction electrode is inhibited, so that carbon dioxide is not supplied to the three-phase interface and the carbon dioxide reduction reaction does not proceed. Therefore, in this example, 200 nm is selected as the thickness of the reduction electrode 5 which is not significantly affected by transfer immobility of electrons and diffusion inhibition of carbon dioxide. The reduction electrode 5 is an aggregate of particles and has interparticle voids.

The gas reduction sheet 20 may be produced by any method other than the above-mentioned electroless plating method, such as a method of forming a thin film on the ion exchange membrane by an electroless plating method and then using an electrolytic plating method, or a method of using physical vapor deposition and chemical vapor deposition which are dry plating methods.

Electrochemical Measurement and Gas/Liquid Production Amount Measurement

The oxidation tank 1 is filled with the aqueous solution 3. A substrate having a NiO co-catalyst thin film formed thereon was obtained by epitaxially growing a GaN thin film, which was an n-type semiconductor, and AlGaN on a sapphire substrate in this order, vacuum-depositing Ni thereon, and performing heat treatment, and this substrate was used as the oxidation electrode 2 a. The oxidation electrode 2 a was installed in the oxidation tank 1 so as to be submerged in the aqueous solution 3. The aqueous solution 3 was a 1 mol/L sodium hydroxide aqueous solution. The oxidation tank 1 and the reduction tank 4 are separated by the gas reduction sheet 20 obtained by forming the reduction electrode 5 directly on the ion exchange membrane 6. Further, the oxidation electrode 2 a and the reduction electrode 5 are connected by the lead 7. As the light source 11, a 300 W high-pressure xenon lamp (wavelength 450 nm or more was cut and illuminance was 6.6 mW/cm²) was used. This light source 11 was fixed to irradiate the surface of the oxidation electrode 2 a on which the oxidation co-catalyst of the semiconductor photoelectrode was formed. The light irradiation area of the oxidation electrode 2 a was set to 2.5 cm2.

Helium was flowed from the tube 8 to the oxidation tank 1 at a flow rate of 5 ml/min and a pressure of 0.18 MPa, and carbon dioxide (CO₂) was flowed from a gas input port 12 to the reduction tank 4 at a flow rate of 5 ml/min and a pressure of 0.18 MPa. In this system, a carbon dioxide reduction reaction can proceed at the three-phase interface composed of [ion exchange membrane-copper-gas phase carbon dioxide] in the gas reduction sheet 20. The area of the reduction electrode 5 to which carbon dioxide is directly supplied is about 6.25 cm². After the insides of the oxidation tank 1 and the reduction tank 4 were sufficiently replaced with helium and carbon dioxide respectively, the oxidation electrode 2 a was uniformly irradiated with light using the light source 11. Upon the light irradiation, a current flows between the oxidation electrode 2 a and the reduction electrode 5. The gas in the oxidation tank 1 and the reduction tank 4 was collected at any time during the light irradiation, and the reaction products were analyzed by gas chromatograph and liquid chromatograph. As a result, it was confirmed that oxygen was generated in the oxidation tank 1, and that hydrogen, carbon monoxide, methane, ethylene, and formic acid were generated in the reduction tank 4. In addition, the current value between the oxidation electrode 2 a and the reduction electrode 5 at the time of the light irradiation was measured using an electrochemical measuring device (1287 Potentiostat Galvanostat, manufactured by Solartron Analytical).

Example 2

In Example 2, as the aqueous solution 3, a 1 mol/L potassium hydroxide aqueous solution was used instead of the sodium hydroxide aqueous solution used in Example 1. Other configurations are the same as those in Example 1.

Example 3

In Example 3, as the aqueous solution 3, a 1 mol/L rubidium hydroxide aqueous solution was used instead of the sodium hydroxide aqueous solution used in Example 1. Other configurations are the same as those in Example 1.

Example 4

In Example 4, as the aqueous solution 3, a 1 mol/L cesium hydroxide aqueous solution was used instead of the sodium hydroxide aqueous solution used in Example 1. Other configurations are the same as those in Example 1.

Example 5 Configuration of Apparatus

FIG. 3 is a diagram illustrating a configuration of a carbon dioxide gas phase reduction apparatus 100 according to Example 5. This carbon dioxide gas phase reduction apparatus 100 is an improved invention of the related art 2, and is an apparatus for an electrolytic reduction reaction of carbon dioxide at a reduction electrode (electrolytic reduction reaction apparatus). Hereinafter, it is abbreviated as the gas phase reduction apparatus 100.

As illustrated in FIG. 3, the gas phase reduction apparatus 100 includes an oxidation tank 1 and a reduction tank 4 formed by dividing an internal space inside a housing into two. Specifically, the gas phase reduction apparatus 100 includes the oxidation tank 1 filled with an aqueous solution 3, an oxidation electrode 2 b formed from a metal inserted into the oxidation tank 1, and the reduction tank 4 that is adjacent to the oxidation tank 1 and is filled with carbon dioxide or a gas containing carbon dioxide.

A gas reduction sheet 20 is disposed between the oxidation tank 1 and the reduction tank 4. The gas reduction sheet 20 is obtained by laminating a reduction electrode 5 made from a metal layer on an ion exchange membrane 6. The ion exchange membrane 6 is disposed on the oxidation tank 1 side, and the reduction electrode 5 is disposed on the reduction tank 4 side. A power source 9 for operating the gas phase reduction apparatus 100 is connected to the oxidation electrode 2 b and the reduction electrode 5 by a lead 7. The oxidation electrode 2 b is, for example, platinum, gold, silver, copper, indium, or nickel. The reduction electrode 5 b, the aqueous solution 3, and the ion exchange membrane 6 are the same as those in Example 1. The gas reduction sheet 20 is also produced by the same procedure as in Example 1.

Further, as illustrated in FIG. 7, similarly to Example 1, the metal wire 13 is connected to the reduction electrode 5 by the silver paste 14 and fixed with an adhesive applied onto the silver paste 14. The contact area between the reduction electrode 5 and the current collector (metal wire 13 and silver paste 14) is about 0.87 cm², which corresponds to about 14% of the area of the reduction electrode 5.

Electrochemical Measurement and Gas/Liquid Production Amount Measurement

The oxidation tank 1 is filled with the aqueous solution 3. Platinum (manufactured by Nilaco Corporation) was used for the oxidation electrode 2 b, and the oxidation electrode 2 b was placed in the oxidation tank 1 so that a surface area of about 0.55 cm² was submerged in the aqueous solution 3. The oxidation tank 1 and the reduction tank 4 are separated by the gas reduction sheet 20 obtained by directly forming the reduction electrode 5 on the ion exchange membrane 6. The aqueous solution 3 was a 1 mol/L sodium hydroxide aqueous solution.

Helium was supplied from the tube 8 to the oxidation tank 1 at a flow rate of 5 ml/min and a pressure of 0.18 MPa, and carbon dioxide was supplied to the reduction tank 4 from the gas input port 12 at a flow rate of 5 ml/min and a pressure of 0.18 MPa. In this system, a carbon dioxide reduction reaction can proceed at the three-phase interface composed of [ion exchange membrane-copper-gas phase carbon dioxide] in the gas reduction sheet 20. The area of the reduction electrode 5 to which carbon dioxide is directly supplied is about 6.25 cm². After the insides of the oxidation tank 1 and the reduction tank 4 were sufficiently replaced with helium and carbon dioxide respectively, the power source 9 was connected between the oxidation electrode 2 b and the reduction electrode 5 by the lead 7, and a voltage of 2.5 V was applied to flow a current. The gas in the oxidation tank 1 and the reduction tank 4 was collected at any time during the voltage application, and the reaction products were analyzed by gas chromatograph and liquid chromatograph. As a result, it was confirmed that oxygen was generated in the oxidation tank 1, and that hydrogen, carbon monoxide, methane, ethylene, and formic acid were generated in the reduction tank 4. Further, the current value between the oxidation electrode 2 b and the reduction electrode 5 when a voltage of 2.0 V was applied was measured using an electrochemical measuring device.

Example 6

In Example 6, as the aqueous solution 3, a 1 mol/L potassium hydroxide aqueous solution was used instead of the sodium hydroxide aqueous solution used in Example 5. Other configurations are the same as those in Example 5.

Example 7

In Example 7, as the aqueous solution 3, a 1 mol/L rubidium hydroxide aqueous solution was used instead of the sodium hydroxide aqueous solution used in Example 5. Other configurations are the same as those in Example 5.

Example 8

In Example 8, as the aqueous solution 3, a 1 mol/L cesium hydroxide aqueous solution was used instead of the sodium hydroxide aqueous solution used in Example 5. Other configurations are the same as those in Example 5.

Example 9

In Example 9, as a current collector for the reduction electrode 5, a silver metal plate 15 having a thickness of 2.0 mm as illustrated in FIG. 8 was used instead of the metal wire 13 and the silver paste 14 used in Examples 1 to 4. The metal plate 15 was pressed against a 1.8 mm wide peripheral portion of the reduction electrode 5 of the gas reduction sheet 20 to collect current. The metal plate 15 and the reduction electrode 5 are tightened by bolts and nuts and are in contact with each other. If the metal plate 15 is thin, it will be distorted during tightening, and if it is thick, the volume of the reaction cell will increase. Therefore, the thickness of the reduction electrode 5 is preferably in the range of about 1.0 mm to 3.0 mm. In the present example, about 2.0 mm was selected. The metal plate 15 and the oxidation electrode 2 a are electrically connected by the lead 7. The contact area between the metal plate 15 and the reduction electrode 5 is about 0.87 cm², which corresponds to about 14% of the area of the reduction electrode 5. Other configurations are the same as those in Example 2.

Example 10

In Example 10, as a current collector for the reduction electrode 5, a metal mesh 16 in which silver wires with a diameter of 0.2 mm intersected vertically and horizontally at intervals of 2.5 mm as illustrated in FIG. 9 was used instead of the metal wire 13 and the silver paste 14 used in Examples 1 to 4, and the metal mesh 16 was pressed against the surface of the reduction electrode 5 of the gas reduction sheet 20 to collect current. At a 5.0 mm wide outer peripheral portion of the reduction electrode 5, the reduction electrode 5, the metal mesh 16, and the metal plate 15 are laminated. These three members are tightened by bolts and nuts and are in contact with each other. The metal plate 15 and the oxidation electrode 2 a are electrically connected by the lead 7. The contact area between the metal mesh 16 and the reduction electrode 5 is about 0.9 cm², which corresponds to about 14% of the area of the reduction electrode 5 (aperture ratio about 86%). Other configurations are the same as those in Example 2.

Example 11

In Example 11, as a current collector for the reduction electrode 5, a metal mesh 16 in which silver wires with a diameter of 0.2 mm intersected vertically and horizontally at intervals of 1.2 mm as illustrated in FIG. 9 was used instead of the metal wire 13 and the silver paste 14 used in Examples 1 to 4, and the metal mesh 16 was pressed against the surface of the reduction electrode 5 of the gas reduction sheet 20 to collect current. At a 5.0 mm wide outer peripheral portion of the reduction electrode 5, the reduction electrode 5, the metal mesh 16, and the metal plate 15 are laminated. These three members are tightened by bolts and nuts and are in contact with each other. The metal plate 15 and the oxidation electrode 2 a are electrically connected by the lead 7. The contact area between the metal mesh 16 and the reduction electrode 5 is about 2.0 cm², which corresponds to about 32% of the area of the reduction electrode 5 (aperture ratio about 68%). Other configurations are the same as those in Example 2.

Example 12

In Example 12, as a current collector for the reduction electrode 5, a metal mesh 16 in which silver wires with a diameter of 0.2 mm intersected vertically and horizontally at intervals of 0.6 mm as illustrated in FIG. 9 was used instead of the metal wire 13 and the silver paste 14 used in Examples 1 to 4, and the metal mesh 16 was pressed against the surface of the reduction electrode 5 of the gas reduction sheet 20 to collect current. At a 5.0 mm wide outer peripheral portion of the reduction electrode 5, the reduction electrode 5, the metal mesh 16, and the metal plate 15 are laminated. These three members are tightened by bolts and nuts and are in contact with each other. The metal plate 15 and the oxidation electrode 2 a are electrically connected by the lead 7. The contact area between the metal mesh 16 and the reduction electrode 5 is about 3.8 cm², which corresponds to about 61% of the area of the reduction electrode 5 (aperture ratio about 29%). Other configurations are the same as those in Example 2.

Example 13

In Example 13, as a current collector for the reduction electrode 5, a metal mesh 16 in which silver wires with a diameter of 0.2 mm intersected vertically and horizontally at intervals of 0.4 mm as illustrated in FIG. 9 was used instead of the metal wire 13 and the silver paste 14 used in Examples 1 to 4, and the metal mesh 16 was pressed against the surface of the reduction electrode 5 of the gas reduction sheet 20 to collect current. At a 5.0 mm wide outer peripheral portion of the reduction electrode 5, the reduction electrode 5, the metal mesh 16, and the metal plate 15 are laminated. These three members are tightened by bolts and nuts and are in contact with each other. The metal plate 15 and the oxidation electrode 2 a are electrically connected by a lead. The contact area between the metal mesh 16 and the reduction electrode 5 is about 5.6 cm², which corresponds to about 90% of the area of the reduction electrode 5 (aperture ratio about 10%). Other configurations are the same as those in Example 2.

Example 14

In Example 14, as a current collector for the reduction electrode 5, a metal plate 15 having a thickness of 2.0 mm as illustrated in FIG. 8 was used instead of the metal wire 13 and the silver paste 14 used in Examples 1 to 4, and the metal plate 15 was pressed against a 1.8 mm wide outer peripheral portion of the reduction electrode 5 of the gas reduction sheet 20 to collect current. The metal plate 15 and the reduction electrode 5 are tightened by bolts and nuts and are in contact with each other. The metal plate 15 and the oxidation electrode 2 a are electrically connected by the lead 7. The contact area between the metal plate 15 and the reduction electrode 5 is about 0.87 cm², which corresponds to about 14% of the area of the reduction electrode. Other configurations are the same as those of Example 6.

Example 15

In Example 15, as a current collector for the reduction electrode 5, a metal mesh 16 in which silver wires with a diameter of 0.2 mm intersected vertically and horizontally at intervals of 2.5 mm as illustrated in FIG. 9 was used instead of the metal wire 13 and the silver paste 14 used in Examples 1 to 4, and the metal mesh 16 was pressed against the surface of the reduction electrode 5 of the gas reduction sheet 20 to collect current. At a 5.0 mm wide outer peripheral portion of the reduction electrode 5, the reduction electrode 5, the metal mesh 16, and the metal plate 15 are laminated. These three members are tightened by bolts and nuts and are in contact with each other. The metal plate 15 and the oxidation electrode 2 a are electrically connected by the lead 7. The contact area between the metal mesh 16 and the reduction electrode 5 is about 0.9 cm², which corresponds to about 14% of the area of the reduction electrode 5 (aperture ratio about 86%). Other configurations are the same as those in Example 6.

Example 16

In Example 16, as a current collector for the reduction electrode 5, a metal mesh 16 in which silver wires with a diameter of 0.2 mm intersected vertically and horizontally at intervals of 1.2 mm as illustrated in FIG. 9 was used instead of the metal wire 13 and the silver paste 14 used in Examples 1 to 4, and the metal mesh 16 was pressed against the surface of the reduction electrode 5 of the gas reduction sheet 20 to collect current. At a 5.0 mm wide outer peripheral portion of the reduction electrode 5, the reduction electrode 5, the metal mesh 16, and the metal plate 15 are laminated. These three members are tightened by bolts and nuts and are in contact with each other. The metal plate 15 and the oxidation electrode 2 a are electrically connected by the lead 7. The contact area between the metal mesh 16 and the reduction electrode 5 is about 2.0 cm², which corresponds to about 32% of the area of the reduction electrode 5 (aperture ratio about 68%). Other configurations are the same as those in Example 6.

Example 17

In Example 17, as a current collector for the reduction electrode 5, a metal mesh 16 in which silver wires with a diameter of 0.2 mm intersected vertically and horizontally at intervals of 0.6 mm as illustrated in FIG. 9 was used instead of the metal wire 13 and the silver paste 14 used in Examples 1 to 4, and the metal mesh 16 was pressed against the surface of the reduction electrode 5 of the gas reduction sheet 20 to collect current. At a 5.0 mm wide outer peripheral portion of the reduction electrode 5, the reduction electrode 5, the metal mesh 16, and the metal plate 15 are laminated. These three members are tightened with bolts and nuts and are in contact with each other. The metal plate 15 and the oxidation electrode 2 a are electrically connected by the lead 7. The contact area between the metal mesh 16 and the reduction electrode 5 is about 3.8 cm², which corresponds to about 61% of the area of the reduction electrode (aperture ratio about 29%). Other configurations are the same as those in Example 6.

Example 18

In Example 18, as a current collector for the reduction electrode 5, a metal mesh 16 in which silver wires with a diameter of 0.2 mm intersected vertically and horizontally at intervals of 0.4 mm as illustrated in FIG. 9 was used instead of the metal wire 13 and the silver paste 14 used in Examples 1 to 4, and the metal mesh 16 was pressed against the surface of the reduction electrode 5 of the gas reduction sheet 20 to collect current. At a 5.0 mm wide outer peripheral portion of the reduction electrode 5, the reduction electrode 5, the metal mesh 16, and the metal plate 15 are laminated. These three members are tightened by bolts and nuts and are in contact with each other. The metal plate 15 and the oxidation electrode 2 a are electrically connected by the lead 7. The contact area between the metal mesh 16 and the reduction electrode 5 is about 5.6 cm², which corresponds to about 90% of the area of the reduction electrode 5 (aperture ratio about 10%). Other configurations are the same as those in Example 6.

Comparative Example 1

The configuration of Comparative Example 1 corresponding to Examples 1 to 4 is as illustrated in FIG. 5.

Compared with FIG. 1, the oxidation tank 1 is similar, but the structures of the ion exchange membrane 6 and the reduction tank 4 are different. The oxidation tank 1 and the reduction tank 4 are separated by the ion exchange membrane 6, and the reduction electrode 5 a is submerged in the aqueous solution 3 (3-2) contained in the reduction tank 4. The aqueous solution 3 (3-1) in the oxidation tank 1 was a 1 mol/l sodium hydroxide aqueous solution, and the aqueous solution 3 (3-2) in the reduction tank 4 was a 0.5 mol/l potassium hydrogen carbonate aqueous solution. A copper plate (manufactured by Nilaco Corporation) having an area of about 6 cm² was used as the reduction electrode 5 a and installed so as to be submerged in the aqueous solution 3 (3-2). Other configurations are the same as those in Example 1. It was confirmed through the analysis of the reaction products that hydrogen, carbon monoxide, methane, ethylene, and formic acid were generated.

Comparative Example 2

FIG. 4 illustrates a configuration of Comparative Example 2 corresponding to Examples 5 to 8.

Compared with FIG. 3, the oxidation tank 1 is similar, but the structures of the ion exchange membrane 6 and the reduction tank 4 are different. The oxidation tank 1 and the reduction tank 4 are separated by the ion exchange membrane 6, and the reduction electrode 5 is submerged in the aqueous solution 3 (3-2) contained in the reduction tank 4. The aqueous solution 3 (3-1) in the oxidation tank 1 was a 1 mol/l sodium hydroxide aqueous solution, and the aqueous solution 3 (3-2) in the reduction tank 4 was a 0.5 mol/l potassium hydrogen carbonate aqueous solution. A copper plate (manufactured by Nilaco Corporation) having an area of about 6 cm² was used as the reduction electrode 5 and installed so as to be submerged in the aqueous solution 3 (3-2). Other configurations are the same as in Example 5. It was confirmed through the analysis of the reaction products that hydrogen, carbon monoxide, methane, ethylene, and formic acid were generated.

Effects of Examples

From the results of gas generation amount measurement, it was confirmed that oxygen was generated in the oxidation tank under all the conditions of Examples 1 to 8 and Comparative Examples 1 and 2. In the reduction tank, hydrogen generation by proton reduction reaction (4H⁺+4e⁻→2H₂), carbon monoxide generation by carbon dioxide reduction reaction (CO₂+2H⁺+2e⁻→CO+H₂O), methane generation (CO₂+8H⁺+8e⁻→CH₄+2H₂O), ethylene generation (2CO₂+12H⁺+12e⁻→C₂H₄+4H₂O), formic acid generation (CO₂+2H⁺+2e⁻→HCOOH) were confirmed.

Effect 1

Table 2 shows the Faraday efficiency of hydrogen generation by proton reduction and substance generation by carbon dioxide reduction at given times in Examples 1 and 5 and Comparative Examples 1 and 2. The Faraday efficiency indicates the ratio of the current value used for the reduction reaction to the current value flowing in the lead during light irradiation or voltage application.

Table 2 shows the Faraday efficiency of hydrogen generation by proton reduction and substance generation by carbon dioxide reduction. The Faraday efficiency indicates the ratio of the current value used for each reduction reaction to the current value flowing between the electrodes during light irradiation or voltage application. The current value used for each reduction reaction can be obtained by converting the measured value of the generated amount of each reduction product into the number of electrons required for the production reaction.

TABLE 2 Hydrogen generation by Substance generation by proton reduction (%) carbon dioxide reduction (%) Example 1 69 22 Comparative 87 2 Example 1 Example 5 62 19 Comparative 70 10 Example 2

Table 2 shows that, as compared with Comparative Examples 1 and 2, in Examples 1 and 5, the Faraday efficiency of substance generation by carbon dioxide reduction was improved, and that the Faraday efficiency of hydrogen generation by proton reduction was decreased. From this, it can be seen that the reduction reaction of carbon dioxide occurs selectively on the surface of the reduction electrode rather than the reduction reaction of protons. This is considered to be because in Examples 1 and 5, the diffusion resistance of carbon dioxide near the copper surface was decreased by directly supplying the carbon dioxide in the gas phase to the copper (reduction electrode) without passing through the aqueous solution.

Regarding Examples 2 and 6 and Examples 9 to 18, the current value between the electrodes one minute after the start of measurement is shown on the left side of Table 3. Table 3 shows the initial current value one minute after the start of light irradiation or the application of voltage.

TABLE 3 Initial current value (mA) Example 2 1.1 Example 9 1.3 Example 10 1.3 Example 11 1.4 Example 12 1.6 Example 13 1.6 Example 6 18 Example 14 20 Example 15 21 Example 16 23 Example 17 24 Example 18 24

Compared with Examples 2 and 6 in which the lead 7 is connected using the silver paste 14, it was found that the current value between the electrodes improved one minute after the start of light irradiation or the application of voltage in Examples 9 to 18 using the method of pressing the metal plate 15 against the outer peripheral portion of the reduction electrode 5 and the method of pressing the metal mesh 16 against the surface of the reduction electrode 5. This is considered to be because in Examples 9 to 18, the current collector can contact the surface of the reduction electrode 5 more uniformly, and the ratio of the metal particles constituting the reduction electrode 5 connected to the current collector is improved, whereby the reaction field is increased.

Further, in Examples 10 to 12 and 15 to 17, the initial current value improved as the contact area between the reduction electrode 5 and the current collector increased. This is considered to be because the ratio of the metal particles connected to the current collector to the metal particles forming the reduction electrode 5 was increased.

Furthermore, in Examples 12 and 13, and Examples 17 and 18, the initial current value is a fixed value. It is conceivable that this is because all of the metal particles forming the reduction electrode in the initial state were connected to the current collector.

Further, in the aperture ratio of the metal mesh 16 used in Examples 9 to 18, it was not confirmed that the diffusion resistance was increased due to the metal mesh 16 inhibiting the diffusion of carbon dioxide. Since the porosity of the reduction electrode 5 is about 5%, the aperture ratio of the metal mesh needs to be 5% or more in order to prevent the diffusion resistance of carbon dioxide from increasing.

Effect 2

Of the measured current values between the oxidation electrode and the reduction electrode, the current value that contributed to the generation of carbon monoxide, methane, ethylene, and formic acid by the carbon dioxide reduction reaction was defined as the current value of the carbon dioxide reduction reaction. Table 4 shows a current retention rate of the carbon dioxide reduction reaction 2.5 hours after the start of light irradiation or the application of voltage in Examples 1 to 8 and Comparative Examples 1 and 2. The current retention rate of carbon dioxide reduction reaction is defined as the ratio of the current value contributing to the carbon dioxide reduction reaction at any time to the current value contributing to the carbon dioxide reduction reaction 10 minutes after the start of light irradiation.

TABLE 4 Current retention rate of carbon dioxide reduction reaction (%) Example 1 46 Example 2 77 Example 3 78 Example 4 76 Comparative Example 1 9 Example 5 59 Example 6 81 Example 7 78 Example 8 77 Comparative Example 2 11

Table 4 shows that the current retention rate of the carbon dioxide reduction reaction was improved in each example, compared with Comparative Examples 1 and 2. This indicates that the life of the carbon dioxide reduction reaction was improved. This is considered to be because the deterioration reaction of the reduction electrode was suppressed by preventing the reduction electrode from coming into contact with the aqueous solution in the examples.

Further, the results regarding different aqueous solutions (sodium hydroxide aqueous solution and potassium hydroxide aqueous solution) in the oxidation tanks of Examples 2 to 4, and 6 to 8 proved that the current retention rate of the carbon dioxide reduction reaction was higher when the potassium hydroxide aqueous solution, the rubidium hydroxide aqueous solution, and the cesium hydroxide aqueous solution were used than when the sodium hydroxide aqueous solution was used.

When the sodium hydroxide aqueous solution was used, a white solid of sodium hydrogen carbonate (NaHCO₃) was precipitated on the Cu surface as the reaction proceeded whereas no precipitate was observed on the surface when the potassium hydroxide aqueous solution, the rubidium hydroxide aqueous solution, or the cesium hydroxide aqueous solution was used. When the sodium hydroxide aqueous solution was used, this precipitate covered the reaction field and thus decreased the current. Sodium hydrogen carbonate is generated when sodium ions (Nat) in the sodium hydroxide aqueous solution move from the oxidation tank to the reduction tank via the ion exchange membrane and react with carbon dioxide (NaOH+CO₂→NaHCO₃).

Therefore, it is conceivable that when a potassium hydroxide aqueous solution containing potassium ion (K⁺), a rubidium hydroxide aqueous solution containing rubidium ion (Rb⁺), and a cesium hydroxide aqueous solution containing cesium ion (Cs⁺), which had an ionic radius larger than that of Na⁺, were used, potassium ions, rubidium ions, and cesium ions could not pass through the ion exchange membrane, which prevented the formation of precipitates. The ionic radius of sodium ion, potassium ion, rubidium ion, and cesium ion are 0.99 Δ, 1.37 Δ, 1.52 Δ, and 1.67 Δ, respectively.

Further, Table 5 shows the current retention rate of the carbon dioxide reduction reaction 2.5 hours after the start of measurement in Examples 2 and 6 and Examples 9 to 18. Table 5 shows the current retention rate of the carbon dioxide reduction reaction 2.5 hours after the start of light irradiation or the application of voltage.

TABLE 5 Current retention rate of carbon dioxide reduction reaction (%) Example 2 77 Example 9 80 Example 10 81 Example 11 83 Example 12 85 Example 13 86 Example 6 81 Example 14 82 Example 15 82 Example 16 84 Example 17 86 Example 18 86

Compared with Examples 2 and 6 in which the lead 7 is connected using the silver paste 14, it was found that the current retention rate of the carbon dioxide reduction reaction 2.5 hours after the start of light irradiation or the application of voltage was improved in Examples 9 to 18 in which the current collector is connected being pressed against the reduction electrode 5. As the reaction proceeds, the reduction reaction product adheres to the surface of the reduction electrode and the inside of the voids. It is conceivable that when the silver paste 14 is used, the current retention rate of the carbon dioxide reduction reaction is lowered because the silver paste 14 and the reduction electrode 5 are easily peeled off due to the influence of the reduction product adhering to the surface. Further, the metal particles constituting the reduction electrode 5 are physically and electrically torn due to the effect in which the reduction electrode 5 reacts with the adhering reduction product and deteriorates, which is considered to be a reason why the current retention rate of the carbon dioxide reduction reaction decreases.

On the other hand, in the case of the connection method using the metal plate 15 or the metal mesh 16 as the current collector as in Examples 9 to 18, the reduction electrode 5 and the current collector are tightened and brought into contact with each other, and thus the current collector does not peel off from the reduction electrode 5 even if the reduction reaction product adheres to the electrode.

Further, in Examples 10 to 13 and 15 to 23, it can be seen that an increase in the contact area between the metal mesh 16 and the surface of the reduction electrode 5 improves the current retention rate of the carbon dioxide reduction reaction. This is considered to be because the current collector can be connected to many metal particles to increase the reaction field, even if the metal particles are electrically torn due to the influence of the reduction product adhering to the surface of the metal particles constituting the reduction electrode 5.

From the above, it is conceivable that the current retention rate of carbon dioxide reduction was improved.

Effects of the Invention

According to the present embodiment, carbon dioxide in the gas phase is directly supplied to the gas reduction sheet obtained by forming the reduction electrode directly on the ion exchange membrane, and thus the diffusion resistance of carbon dioxide in the vicinity of the reduction electrode can be decreased, which can improve the efficiency of the carbon dioxide reduction reaction on the reduction electrode.

Further, according to the present embodiment, only the carbon dioxide in the gas phase is brought into contact with the reduction electrode, it is possible to prevent the reduction electrode from deteriorating due to the reaction with the aqueous solution and improve the life of the carbon dioxide reduction reaction.

Further, according to the present embodiment, any one of a potassium hydroxide aqueous solution, a rubidium hydroxide aqueous solution, and a cesium hydroxide aqueous solution is used as the aqueous solution with which the oxidation tank is filled, that is, an aqueous solution containing ions having a large ionic radius is used, and thus it is possible to suppress the decrease in the reaction area due to solid precipitation on the surface of the reduction electrode and further improve the life of the carbon dioxide reduction reaction.

Further, according to the present embodiment, as a connection method between the reduction electrode 5 and the lead 7, a method of pressing the metal plate 15 against the outer periphery of the reduction electrode 5 and a method of pressing the metal mesh 16 against the surface of the reduction electrode 5 were used. Since the porosity of the reduction electrode 5 is about 5%, current collection can be achieved without inhibition of the diffusion of carbon dioxide by setting the aperture ratio of the metal mesh 16 to 5% or more. With this configuration, sufficient current collection can be achieved by the reduction electrode 5 and the lead 7, which can further improve efficiency and life of the carbon dioxide reduction reaction. It is conceivable that when the area of the reduction electrode 5 is large, the current collecting structure in which the metal mesh 16 is uniformly pressed against the entire electrode surface becomes more effective.

That is, as a connection configuration and method of the lead of the reduction electrode (structure and method of electrode current collection), a configuration of correcting a current by pressing the metal plate 15 or the metal mesh 16 against the surface of the reduction electrode 5 as illustrated in FIGS. 8 and 9 is used. Therefore, even in an electrode having a porous structure such as the reduction electrode 5 formed on the gas reduction sheet 20 in which carbon dioxide gas diffuses, electrons necessary for the reduction reaction can be uniformly and stably supplied to the electrode surface, which can increase the reduction reaction current of carbon dioxide and improve long-term stability.

REFERENCE SIGNS LIST

-   1 Oxidation tank -   2, 2 a, 2 b Oxidation electrode -   3 Aqueous solution -   4 Reduction tank -   5, 5 a, 5 b Reduction electrode -   6 Ion exchange membrane -   7 Lead -   8 Tube -   9 Power supply -   10 Reference electrode -   11 Light source -   12 Gas input port -   13 Metal wire -   14 Silver paste -   15 Metal plate -   16 Metal mesh -   20 Gas reduction sheet -   31, 32 Tank -   41 Plating solution -   42 Reducing agent 

1. A carbon dioxide gas phase reduction apparatus comprising: an oxidation tank including an oxidation electrode; a reduction tank which is adjacent to the oxidation tank and to which carbon dioxide is supplied; and a gas reduction sheet located between the oxidation tank and the reduction tank, wherein the gas reduction sheet is a sheet in which an ion exchange membrane and a reduction electrode are laminated, the ion exchange membrane is located on a side of the oxidation tank, and the reduction electrode is located on a side of the reduction tank, is connected to the oxidation electrode by a lead, and conducts a reduction reaction of the carbon dioxide by a current flowing through the lead.
 2. The carbon dioxide gas phase reduction apparatus according to claim 1, further comprising a light source for irradiating the oxidation electrode with light.
 3. The carbon dioxide gas phase reduction apparatus according to claim 1, further comprising a power source connected to the lead.
 4. The carbon dioxide gas phase reduction apparatus according to claim 1, wherein the oxidation electrode comprises an n-type semiconductor.
 5. A carbon dioxide gas phase reduction method performed in a carbon dioxide gas phase reduction apparatus, the carbon dioxide gas phase reduction apparatus including an oxidation tank including an oxidation electrode, a reduction tank adjacent to the oxidation tank, and a gas reduction sheet located between the oxidation tank and the reduction tank, the gas reduction sheet being a sheet in which an ion exchange membrane and a reduction electrode are laminated, the ion exchange membrane being located on a side of the oxidation tank, the reduction electrode being located on a side of the reduction tank and connected to the oxidation electrode by a lead, the gas phase reduction method comprising: filling the oxidation tank with an aqueous solution; supplying carbon dioxide to the reduction tank, and flowing an electric current through the lead to cause a reduction reaction of the carbon dioxide at the reduction electrode.
 6. The gas phase reduction method for carbon dioxide according to claim 5, wherein the aqueous solution is any one of a potassium hydroxide aqueous solution, a rubidium hydroxide aqueous solution, and a cesium hydroxide aqueous solution. 